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July 16, 2009

Physicists Say Many Asteroids May Be Retired Comets

by Sam Savage

According to a team of astrophysicists, a number of the heavenly bodies drifting about in the asteroid belt between Mars and Jupiter were previously comets that had been ejected from their orbit around planets.

Their report challenges the commonly held notion that most of the asteroids are left-overs from "proto-planetary disks," or the thick gas ring that surrounds young stars.

The odd make-up of the orbiting rocks has long tickled the curiosity of scientists, with their strange mixtures of ice and various types of rock. The existence of igneous rock in particular"”formed when liquid-hot magma is cooled"”has led researchers in the field to suspect that the rocks may have diverse origins.

The report, published in Wednesday's edition of the science journal Nature, says that the apparent mixed lineage of the asteroids can be explained by the fact that a significant portion of them were once space-wandering comets that found their final resting place in the immense asteroid belt.

The team of French and U.S. researchers supported their theory by devising a mathematical model of the early development of the Solar System at time when the planets were just beginning to form.

If their model is correct, Jupiter, Saturn, Uranus and Neptune were at that time in relatively close proximity to each other"”between roughly 450 million 1.4 billion miles apart.

Beyond the four giant planets was a disk-shaped mass of comets, known as trans-Neptunian objects, between 1.5 and 3 billion miles from the Sun. As the four massive planets continued to grow, their orbits around the sun became unstable until roughly 600 million years ago Uranus and Neptune spun out of their orbits.

The two planets then slammed into the mass of comets, many of which scattered throughout the entire Solar System, while a smaller number of them were captured by the weak gravitational forces of the asteroid belt where they still remain in orbit.

"This last major event of planet formation appears to have affected nearly every nook and cranny of the solar system," explained the report's lead author Dr. Hal Levison of Southwest Research Institute (SwRI).

Important evidence for the cosmic calamity was first uncovered in samples returned from the Moon by the Apollo astronauts. Tests on the samples indicated that an ancient hail storm of large asteroids and comets had pelted the Moon's surface.

Researchers now understand that this event was not limited solely to the Moon. A number of planetary bodies, including Earth, were also affect by it.

"The existence of life on Earth, as well as the conditions that made our world habitable for us, are strongly linked to what happened at this distant time," said Dr. David Nesvorny, also of SwRI.

Matthieu Gounelle of France's National Centre for Scientific Research (CNRS) has described the discovery as a "paradigm shift."

"The asteroid belt is not just a leftover from the formation of the Solar System, but also that of violent phenomena" including the great planetary migration," he said in a press release.

If the group's model is correct, it would indicate that ancient asteroids and comets in our Solar System are more closely related that scientists had previously expected. It could also help to clear up the controversy surrounding the origins of micrometeorites, the mini balls of rock that manage to make it through the Earth's atmosphere.

Micrometeorites typically have higher levels of carbon-based matter and are more brittle in texture than meteorites, which scientists say would make sense if they are derived from comets.